There's nothing you can really do on this island, but it's what's inside the island that counts. In order to accomplish anything, pick up and smash the giant stone head (with the help of your Power Bracelets, of course) to reveal a hidden entrance.

Inside this cave, you will find a battle dungeon. Clear all the rooms (and if you need to, steal pendants from the Bokoblins). After you've cleared all the rooms, exit back out to the central arena and you'll encounter two Darknut knights. Defeat them to unlock the door to the Triforce Chart chamber.

Inside, stand on the blue emblem and play the Wind's Requiem to make the treasure chest appear. Open it to get Triforce Chart 7 (GameCube Version) or Triforce Shard 7 (Wii U Version).

After you've finished the battle dungeon, sail east and find a lookout platform. Destroy the cannons (either with your boat's own cannon or when you're up there by using bombs on top of the platform) and defeat the enemies. A treasure chest with a Piece of Heart will appear.